Recently, electrophotography has been employed in the field of commercial printing and there has been required to be capable of stably supplying images of high quality. To meet such demand, there is required a two-component developer which is difficult to be affected deterioration due to printing or environment variation at the time of printing. In response thereto, there was disclosed a two-component developer, in which an acyclic methacrylate was used for a resin covering layer of a carrier to attain electrostatic charge even under an environment of high temperature and high humidity and to inhibit environmental difference, achieving stabilization of image quality, as described in, for example, JP 3691085 B.
The thus disclosed two-component developer caused a spent phenomenon in which a toner or an external additive adhered to the carrier surface by mechanical stress within a developing device or charge providing capability of a carrier was lowered by abrasion of a resin covering layer, and when performing printing of a large number of sheets, it sometimes became difficult to provide an optimal amount of electrostatic charge to a toner. To overcome such a spent phenomenon, there was applied a technique in which the resin covering layer was caused to be gradually abraded by grinding through mechanical stress such as stirring, whereby the surface layer of the carrier onto which a toner or an external additive adheres was refreshed and electrification capability equivalent to that at the initial stage of printing was maintained.
However, when the resin covering layer become thin by grinding, the electric resistance of the resin covering layer is lessened, rendering it difficult to maintain electrostatic electrification capability and resulting in a lowering of the electrostatic charge, therefore, life of the thus disclosed two-component developer was limited.
There was also disclosed a two-component developer wherein a resin in which a monomer containing a cycloalkyl group was allowed to copolymerize with a nitrogen-containing acrylic monomer (acrylic monomer containing an amino group or its derivatives), was used for a resin covering layer of a carrier, and enhanced electrostatic electrification capability was achieved by introduction of a nitrogen element to the resin covering layer, as described in, for example, JP 2009-300531.
Such a two-component developer has became feasible to maintain high electrostatic charge at the initial stage of printing, however, when performing a large number of prints, abrasion of the resin covering layer results in a lowering of electrostatic charge, so that life of the developer was limited.